Indoor mould growth is associated with poor indoor air quality (IAQ) and increased upper respiratory disease. Mould growth in homes is primarily dependent on the presence of moisture in the indoor environment and the type of building materials used. Traditional interior sheathing materials, such as gypsum drywall, contain an organic paper coating which is a known active site for mould growth when exposed to elevated moisture levels. In conditions in which interior moisture levels are challenging to control (e.g., due to strong weather-induced vapour drive, high internal moisture loads from overcrowding, lack of availability and/or use of ventilation systems), substitution of interior sheathing materials may reduce the prevalence of mould. As such, the introduction of novel materials that can effectively limit mould growth in high moisture environments is of critical importance. Magnesium oxide (MgO) board is one material that is proposed as a potentially mould-resistant, inorganic replacement for interior gypsum board. While this material has been used in building in China, Russia, Scandinavia, and parts of the USA, there is little to no research exploring its mould growth sensitivity. Through this work, two MgO board products and gypsum board samples were placed in a constant climate chamber (85% RH and 30 °C) and mould growth was observed over time. Both MgO products were observed to have a higher resistance to mould growth, indicating their potential as a replacement for gypsum wallboard in unfavorable indoor environments to reduce mould growth.